1. Field of the Invention
The subject invention relates generally to a variable compression ratio engine in which the compression ratio in a cylinder for an internal combustion engine is adjusted while the engine is running, and more specifically toward an improved piston and connecting rod arrangement for dynamically varying the engine compression ratio.
2. Related Art
Gasoline engines have a limit on the maximum pressure that can be developed during the compression stroke. When the fuel/air mixture is subjected to pressure and temperature above a certain limit for a given period of time, it autoignites rather than burns. Maximum combustion efficiency occurs at maximum combustion pressures, but in the absence of compression-induced autoignition that can create undesirable noise and also do mechanical damage to the engine. When higher power outputs are desired for any given speed, more fuel and air must be delivered to the engine. To achieve greater fuel/air delivery, the intake manifold pressure is increased by an additional opening of a throttle plate or by the use of turbochargers or superchargers, which also increase the engine inlet pressures. For engines already operating at peak efficiency/maximum pressure, however, the added inlet pressures created by turbochargers and superchargers would over compress the combustion mixtures, thereby resulting in autoignition, often called knock due to the accompanying sound produced. If additional power is desired when the engine is already operating with combustion pressures near the knock limit, the ignition spark timing must be retarded from the point of best efficiency. This ignition timing retard results in a loss of engine operating efficiency and also an increase of combustion heat transferred to the engine. Thus, a dilemma exists: the engine designer must choose one compression ratio for all modes. A high compression ratio will result in optimal fuel efficiency at light load operation, but at high load operation, the ignition spark must be retarded to avoid autoignition. This results in an efficiency reduction at high load, reduced power output, and increased combustion heat transfer to the engine. A lower compression ratio, in turn, results in a loss of engine efficiency during light load operation, which is typically a majority of the operating cycle.
To avoid this undesirable dilemma, the prior art has taught the concept of dynamically reducing an engine compression ratio whenever a turbocharger or supercharger is activated to satisfy temporary needs for massive power increases. Thus, using variable compression ratio technology, the compression ratio of an internal combustion engine can be set at maximum, peak pressures in non-turbo/super charged modes to increase fuel efficiency while the engine is operating under light loads. However, in the occasional instances when high load demands are placed upon the engine, such as during heavy acceleration and hill climbing, the compression ratio can be lowered, on the fly, to accommodate an increase in the inlet pressure caused by activation of a turbocharger or supercharger. In all instances, compression-induced knock is avoided, and maximum engine efficiencies are maintained.
Various attempts to accomplish dynamic variable compression ratios in an internal combustion engine have been proposed. For example, the automobile company SAAB introduced a variable compression ratio engine at the Geneva Motor Show in the year 2000. The SAAB design consisted of a monoblock cylinder head and a separate crankshaft/crankcase assembly. The monoblock head was connected by a pivot to the crankshaft/crankcase assembly, so that a small (e.g., 4°) relative movement was permitted, which movement was controlled by a hydraulic actuator. The SAAB mechanism enabled the distance between the crankshaft center line and the cylinder head to be varied.
Other attempts to accomplish dynamic variable compression ratios have included an effective lengthening/shortening of the connecting rod, which joins the reciprocating piston to a rotating crankshaft. Among the myriad designs which favor adjusting the length of a connecting rod, some are proposed in which an eccentric wristpin connection is provided at the articulating joint between the small end of the connecting rod and the piston. Examples of eccentric wristpin constructions may be found in U.S. Pat. No. 2,427,668 to Gill, issued Sep. 23, 1947, and U.S. Pat. No. 4,687,348 to Naruoka et al., granted Aug. 18, 1987, and also U.S. Pat. No. 4,864,975 to Hasegawa, granted Sep. 12, 1989.
As will be appreciated by those skilled in the art, the connecting rod generates inertial forces when accelerated during cyclic operation in an internal combustion engine. All prior art connecting rods that adjust length through an eccentric bushing at the rod's small end rely on hydraulic columns of oil piped through the connecting rod. A particular shortcoming in all such prior art examples arises when the oil contained inside the connecting rod is directly affected by connecting rod accelerations. Actuation forces transmitted through medium of hydraulic oil are decreased when the connecting rod is accelerated in the opposite direction and substantially increased when accelerated in the same direction. Included gas bubbles in the hydraulic oil thus may create unpredictable reactions, especially if multiple columns of oil are being actuated in timed sequences to move various interrelated latching elements. For example, in a hypothetical prior art engine with 100 mm stroke and a 150 mm long column of oil in the connecting rod, at 6000 RPM the 1st order acceleration on that column of oil at TDC and BDC calculates to 19,739 m/s2. Assuming the oil in that column has a density of 0.9 g/cm3, the pressure difference from one end of the oil column to the other end would be 386 psi.
Some prior art examples employ two columns of oil separately piped through the connecting rod. The two oil columns rely on a differential in pressure at the small (piston) end of the connecting rod to actuate a latch mechanism, but the two columns have different masses due to a difference of oil aeration, or the presence of a metal locking pin in one of the columns, extremely large pressure differentials will be needed at the large (crank) end of the connecting rod to achieve reliable function of the latch mechanism.
Accordingly, there is a need in this art for a reliable method to transmit an energizing force to the latch actuator that is mechanically isolated from the acceleration fields of the connecting rod such that inertial accelerations experienced by the connecting rod do not influence the force transmitter.